1. Field of the Invention
The present invention relates to an image forming apparatus, and more specifically, to a device for measuring a size of a record medium used in the image forming apparatus, such as copiers and laser printers.
2. Description of the Related Art
The image forming apparatuses that include the copier, the laser printer, etc. have each an image formation unit. The image formation unit has a latent image carrier for carrying a latent image and a developing device that renders the latent image visible as a developer image by giving a developer to the latent image carrier. Moreover, the image forming apparatus has transfer means for transferring the developer image by the developing device onto record material being conveying to a predetermined direction. Furthermore, the image forming apparatus has a fixing device that fixes the developer image on the record material by heating and pressurizing the record material to which the developer image is transferred by the transfer means under predetermined fixing processing conditions.
Conventionally, in such an image forming apparatus, as means for detecting a leading edge and a trailing edge of the record material, the size of the record material is found and existence/absence of the record material is detected by a combination of an optical sensor and an actuator (e.g., a flag) as a unit for detecting the leading edge and the trailing edge of the record material.
However, in order to increase a conveyance speed to a higher speed and thereby shorten a first print output time, there is a device for conveying the record material with as narrower a conveyance interval (a distance between pieces of the record material) as possible. In such an apparatus, it is conceivable that the leading edge of the record material is detected by a combination of the optical sensor and the actuator and the trailing edge is detected by a combination of a sensor for record medium discrimination and an LED (light-emitting diode) disposed facing the sensor for record medium discrimination across the record medium, in which the sensor receives transmitted light passing through the record material.
In detecting the trailing edge of the record medium, when the conveyance speed of the record material becomes fast; after the trailing edge of the record medium escaped from the flag, there flapping occurs slightly until the flag returns to an initial position. Therefore, it takes longer time until the trailing edge detection is established as the conveyance speed becomes faster.
Therefore, if the trailing edge of the record material is detected using the sensor for record medium discrimination at a timing at which the sensor receives light from the LED disposed at the opposed position, a time until the detection is established becomes short compared with a case of using a flag etc.
Technical contents related to the conventional technology as described above are disclosed. For example, Japanese Patent Laid-Open No. 8-137340 describes, as a sensor for detecting the leading edge of the record material, a construction that uses a transmission member that operates when the leading edge of the record material abuts against it and an optical sensor for detecting an operational state of the transmission member. It further describes that by the operation of the transmission member, the leading edge of the record material is detected and it is detected whether the record material is a transparent feed form according to whether the record material transmits the light by the optical sensor.
However, in the case where trailing edge detection of the record material is performed by means of the quantity of transmitted light, it is also expected that detection accuracy in the trailing edge detection of the record material may be lowered due to failure or degradation of the LED making the transmitted light, due to failure of the discrimination sensor, or due to toner, dust, etc. adhered on the optical path of the transmitted light. Moreover, as a result, there is also a conceivable possibility that the size of the record material cannot be found.